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Zinc and aluminum have been used as steel coatings since the early 1900s, with early application of thermally sprayed metal coatings (TSMCs) to bridge structures in the 1930s. TSMCs have been used widely in the European bridge industry, the U.S. Navy, and on offshore oil exploration drill platforms for quite some time. TSMCs of zinc, alu- minum, and their alloys can offer substantial advantages when compared with other coatings typically used to protect steel pilings. Organic coatings can fail prematurely as a result of corrosion progression from coating defects. Transportation, handling, installation, or simple long-term material deterioration may cause these defects. TSMCs offer advantages in generally higher mechanical damage resistance, low self-corrosion rates, and the ability to provide steel corrosion control via cathodic protection at coat- ing defects. The objective of this research was to develop a guide for highway agency personnel on the selection and use of TSMCs on highway pilings that would be suit- able as an AASHTO reference. The extensive body of information on TSMCs, including existing guides, was researched so as not to repeat basic research and development work or conflict with industry standards where those standards are applicable. Applicable portions of exist- ing materials were used in the preparation of the guide. This study also sought to resolve any issues that were unclear concerning the use of TSMCs on steel pilings. These issues have included sealer materials, the effects of abrasive mixes, standoff distances, gun-to-surface angles, steel hardness, edges, coating defects, and surface contamination. The study confirmed the positive impact of the use of sealers and that all of the seal- ers tested in the study would be beneficial in improving performance. Tests on grit and shot/grit mixtures show that 100-percent grit provides the best adhesion, followed by the shot/grit mixtures, and then by 100-percent shot. Further research into parameters for angularity and its effect on performance is recommended. A simple field-friendly measurement technique for angularity was not found in this study. Further work is rec- ommended to determine if surface profilometer measurements of peak count (RPC) and root-mean-square (RQ) (these variables are further described in Table 7) can be used effectively and, if so, what the optimum values should be. Meanwhile, classifications of abrasive angularity should be used. SUMMARY THERMALLY SPRAYED METAL COATINGS TO PROTECT STEEL PILINGS: FINAL REPORT AND GUIDE
The study was able to define the effect of hardness with carbon steel and high-strength, low-alloy (HSLA) steel on adhesion and coating performance as well as the effects of sharp edges, defects, and surface contamination on the coating. Areas of further research are recommended, including establishment of a definition of acceptable limits of angularity and a standard for angularity. Continued monitoring of the test panels currently in immersion and alternate (cyclic) immersion testing at Corrproâs Ocean City facility is also recommended. The Thermally Sprayed Metal Coating Guide, the primary product of this studyâs research, includes all of the infor- mation gathered from the literature, industry research, and laboratory testing. 2
